The present invention relates to a image forming apparatus that is represented by apparatuses, such as an electrophotographic-type color electrophotocopying apparatus and a color printer that form color images by superimposing monochromatic images.
Many color image forming apparatuses have employed the following method: four monochromatic-image forming units, each composed of a photosensitive unit and a developing unit, are arranged in series, and a yellow (Y) image, a magenta (M) image, a cyan (C) image, and a black (B) image for reinforcing light and darkness formed by the respective image forming units are laid one on top of another in sequence on a sheet material transported by a transfer belt provided along the image forming units.
Four monochromatic-image forming units, each composed of a photosensitive unit and a developing unit, are arranged in series so as to correspond to a yellow image, a magenta image, a cyan image, and a black image, or a specific number of images defined by subtractive primaries. The transfer belt transports a sheet material made of a transparent resin sheet for sheet paper or an overhead projector. One known method of laying four images one on top of another is such that four images are transferred to an intermediate transferring unit and superimposed one on top of another on the intermediate transfer unit and the superimposed images are transferred to a sheet material at a time.
In these color image forming apparatuses, four color images (four images) are required to be accurately superimposed. Therefore, various types of control are employed to obtain accurately superimposed images.
For example, the control includes the photosensitive unit peripheral speed control and belt peripheral speed control. The photosensitive unit peripheral speed control controls a drum-driving motor to rotate at a constant speed so that a photosensitive-unit peripheral speed, by which an arbitrary point on a peripheral surface of the photosensitive unit provided in each of four photosensitive units is moved, is the same as the belt peripheral speed, by which an arbitrary point on a sheet material transfer belt rotated by a belt driving motor is moved. The belt peripheral speed control detects the rotation speed of a transferring-belt driving motor and thereby controls it to be constant so that the photosensitive-unit peripheral speed is the same as that of the belt. Also, correction is included in the control to be performed for spacings at which portions where the individual photosensitive units of the four image forming units in contact with the sheet material transfer belt. The correction is performed by changing image forming timing for portions where the images are superimposed.
However, in actual operation, it is difficult to obtain a superimposed image that is completely free of deviation for various reasons. The reasons include positional deviations occurring when exposure light is incident on the individual photosensitive units, deviations in pitch of the photosensitive units (image forming units), slippage occurring between a driving roller for driving the sheet material transfer belt and the sheet material transfer belt, the variation in the peripheral speed of the sheet material transfer belt because of changes in the diameter of the driving roller due to thermal expansion.
For these reasons, a run-in control sequence for converging color deviations and an image-density control sequence are also included in the control. The run-in control sequence converges color deviations occurring when images are superimposed. This control is carried out at power-on time and using warm-up time after a cover or the like is opened or shut after sheet materials are jammed or stacked in the apparatus. The image-density control sequence serves to maintain the image density (toner adhesion amount) even when characteristics vary according to the variation in temperature and aged deteriorations.
With the described various corrective control operations being provided, however, color deviation (positional deviation in the superimposed image) occurs. This color deviation occurs when the difference occurs between the peripheral speed of the sheet material transfer belt and the speed of transportation of sheet materials that are transported by an aligning roller toward the sheet material transfer belt.
For example, when the transportation speed of the aligning roller is lower than the peripheral speed of the sheet material transfer belt, the aligning roller causes a load to exert on the sheet material transported by the sheet material transfer belt. The load is exerted in the direction opposing the direction in which the sheet material is transported; therefore, the sheet material is pulled in the aforementioned direction, color deviation occurs on the whole of the sheet material. Also, jitter is caused because of influence of oscillation in paper-feed driving system, which is transferred from the aligning roller.
In contrast, when the transportation speed of the aligning roller is higher than the peripheral speed of the sheet material transfer belt, great deflection occurs on the sheet material. The deflection occurs in a space defined by upper and lower guides provided so as to sandwich the sheet material from the upper and lower sides. That is, the deflection occurs in front and back portions in the direction in which the sheet material is transported between the aligning roller and a roller provided for electrically charging the sheet material is electrostatically attracted onto the sheet material transfer belt. When the deflection of the sheet material increases to a level that cannot be incorporated in the aforementioned space, the deflection extends and thereby causes the sheet material to shake (or, to wave) in the direction in which the sheet material positioned on the sheet material transfer belt is pushed. This causes the position of the sheet material placed on the sheet material transfer belt to deviate, thereby causing color deviation as in the earlier case.
As described above, since the speed at which the aligning roller is required to transport the sheet material has the narrow proper speed range, it is undesirable that the transportation speed of the aligning rollers be faster or slower than the proper speed. Moreover, in adjusting the transportation speed of the sheet material, it is undesirable that the speed be adjusted many times until it has converged at a specific value.
In a monochromatic (e.g., black-only) image forming apparatus, colors need not be superimposed. Moreover, a monochromatic image forming apparatus involving a digital process subjects gradation to a binarization process to express gradation in the density of pixels, which makes jitters less liable to appear. In a color image forming apparatus, however, colors have to be superimposed and the gradation is subjected to a multivalued process, thus forming pixels of different sizes with an equal pitch, which makes jitters conspicuous.
Furthermore, in a small-sized color printer or copying machine, its size is restricted and the distance between the aligning rollers and the image forming section cannot be made longer. The adsorption roller that causes a sheet material to adhere to the transfer belt by suction in the monochromatic mode still remains in the same position even in the color mode, which decreases the slack (or deflection) space.
As a result, a sufficient deflection space for the sheet material cannot be secured between the aligning rollers and the transport rollers for putting the sheet material between them and transporting it and in the sheet material deflection space formed by the top and bottom guide plates located there. In a conventional apparatus of this type, the length that the sheet material can be bent is 2 mm or less and the proper speed range for the A3 longitudinal size 420 mm is as narrow as 0 to 0.48% or less.